A conventional speed changing (transmission) unit is constructed such that the output shaft of an engine is connected to the input shaft of a torque converter and the input shaft of a transmission is connected to the output shaft of the torque converter, while a lock-up clutch is interposed between the input and output shafts of the torque converter so as to operatively connect them to each other via the lock-up clutch.
Such a lock-up clutch has the following five problems from the viewpoints of structure and control.
(1) Problem relating to hydraulic pressure in the torque converter PA0 (II) Problem relating to lock-up off timing PA0 (III) Problem relating to hydraulic pressure in the lock-up clutch for the period of time from lock-up off till lock-up on PA0 (IV) Problem at the time of gradual increasing of hydraulic pressure PA0 (V) Problem relating to hydraulic pressure in the lock-up clutch during normal running of the vehicle
FIG. 25 is a hydraulic circuit diagram which illustrates a conventional control system for the lock-up clutch. This control system includes a lock-up clutch 4, a torque converter 2, a transmission tank 100, a strainer 101, a hydraulic pump 5, a main relief valve 102, a torque converter relief valve 103, a rear brake ring 104, an oil cooler 105, a plurality of oil filters 106, a cooling relief valve 107, a lubricating relief valve 108, a transmission lubricating section 109, a lock-up modulation valve 110 and a solenoid valve 120. The lock-up clutch 4 is controlled with respect to its operative engagement, disengagement from the operatively engaged state and gradual increasing of hydraulic pressure by operating the lock-up modulation valve 110 via the solenoid valve 120.
FIG. 26 is a schematic sectional view which illustrates by way of example the inner structure of such 1 a conventional modulation valve 110 and solenoid valve 120 and FIG. 27 shows a plurality of characteristic diagrams for respective component each illustrating how characteristics vary during speed changing (i.e., shift change) as time elapses.
Specifically, with this control system, during speed changing, a lock-up "OFF" signal is first sent to a solenoid of the solenoid valve 120 (time t.sub.1). As a result, the solenoid valve 120 is brought in an opened state as shown in FIG. 25, whereby pilot hydraulic pressure set by the main relief valve 102 flows through the solenoid valve 120 to displace a piston 130 of the modulation valve 110 in the leftward direction. As the piston 130 displaces a spool 132 in the leftward direction via the piston 131, a port D which has been communicated with the lock-up clutch 4 is closed with the spool 132 and thereby hydraulic oil in the lock-up clutch 4 is drained.
Then, after a predetermined period of lock-up delay time for holding the lock-up clutch 4 in an OFF state elapses, a lock-up "ON" signal is sent to the solenoid of the solenoid valve 120 (time t.sub.2). As a result, the solenoid valve 120 is shifted to a closed state so that working oil which has thrusted the piston 130 is drained via the solenoid valve 120. Thus, pressure of the working oil which has thrusted the piston 130 is reduced to a level of 0 Kg/cm.sup.2, causing the spool 132 to be displaced in the rightward direction by a spring 133 until the valve 110 is brought in an opened state. Consequently, main hydraulic oil flows in an order of A.fwdarw.C.fwdarw.D and is introduced into the lock-up clutch 4. After a filling time t.sub.f elapses, the lock-up clutch 4 is fully filled with hydraulic oil.
At this moment, hydraulic oil which has been introduced through the port D enters a hydraulic chamber 135 between the piston 131 and the spool 132 via an orifice 134 with the result that hydraulic pressure P.sub.v at an outlet of the valve is set to an initial pressure P.sub.o (=Kx/S.sub.1) under a condition that a force induced by hydraulic pressure active on a pressure receiving area S.sub.1 of the piston 131 is balanced with resilient force (kx, where K designates a spring constant and x designate an initial displacement) (see FIG. 27(c)). Thereafter, hydraulic oil which flows through a drilled hole in the valve body 136 to reach a hydraulic chamber behind a piston 139 via an orifice 138 in the cover 137 thrusts the piston 139 in the rightward direction. As the piston 139 moves in the rightward direction, hydraulic pressure in the lock-up clutch 4 is increased.
On the the hand, for the period t.sub.f of filling, hydraulic pressure in the lock-up clutch 4 is held at a level of almost zero but it is gradually increased after it is raised up to initial hydraulic pressure P.sub.a at the same time when the filling is completed (time t.sub.3). When the piston 139 comes in contact with a stopper, increasing of hydraulic pressure is stopped and hydraulic pressure at this time becomes a set pressure P.sub.b for the lock-up valve (time t.sub.4).
Operation of the modulation valve 110 during the speed changing has been described above. The initial hydraulic pressure P.sub.a and a characteristic of gradual increasing of hydraulic pressure derived from the conventional modulation valve 110 are firmly determined depending on the set load Kx of the spring 133, the pressure receiving area S.sub.1 of the piston 131 and other factors. Thus, hydraulic pressure can not be changed arbitrarily.
Further, the foregoing control system is constructed such that the lock-up clutch 4 is fully immersed in a hydraulic chamber of the torque converter 2 and the hydraulic pressure P.sub.t in the torque converter 2 is exerted on the back pressure portion of a piston of the lock-up clutch 4 via a hydraulic passage 115 (see FIG. 25). Accordingly, with this control system, e.g., at the time point t.sub.3 when hydraulic pressure P in the lock-up clutch 4 is raised up to the initial hydraulic pressure P.sub.a, the lock-up clutch 4 is practically operated with a differential pressure P.sub.s (=P.sub.a -P.sub.t) derived by subtracting the hydraulic pressure P.sub.t in the torque converter 4 from the initial clutch pressure P.sub.a, as shown in FIG. 27(d). Thus, when the lock-up clutch 4 is to be brought in an operatively engaged state, it is not filled with hydraulic oil and thereby it fails to be brought in an operatively engaged state, unless hydraulic oil having hydraulic pressure higher than the hydraulic pressure P.sub.t in the torque converter 2 is supplied to the lock-up clutch 4. Here, the differential pressure P.sub.s caused when the clutch hydraulic pressure P is raised up to the initial hydraulic pressure P.sub.a will be hereinafter referred to as an actual initial hydraulic pressure.
For the reason, a conventional apparatus for controlling a lock-up clutch is constructed such that the initial hydraulic pressure P.sub.a is set appreciably higher than the hydraulic pressure P.sub.t in the torque converter 2 and hydraulic pressure for the lock-up clutch 4 is then gradually increased from the initial hydraulic pressure P.sub.a.
However, the hydraulic pressure P.sub.t in the torque converter 2 varies as an engine speed varies. Thus, with the conventional control system, the actual initial hydraulic pressure P.sub.s varies as the hydraulic pressure P.sub.t in the torque converter 2 varies. Accordingly, with the conventional control system, since the initial hydraulic pressure P.sub.a in the lock-up clutch 4 is kept unchanged, e.g., when the hydraulic pressure P.sub.t in the torque converter 2 is increased, the actual initial hydraulic pressure P.sub.s is reduced.
In this manner, with the conventional apparatus, since the actual initial hydraulic pressure P.sub.s to be practically exerted on the lock-up clutch 4 varies as hydraulic pressure in the torque converter 2 varies, the initial hydraulic pressure P.sub.a to be given by the modulation valve 110 is set to such a high pressure that the actual initial hydraulic pressure P.sub.s is not less than zero or is not equal to zero. For the reason, with the conventional apparatus, a period of clutch engaged time (i.e., filling time) fluctuates with the result that a malfunction such as a large magnitude of shock caused by speed changing (shift change)occurs (see FIG. 27(f)).
FIGS. 28(a), (b) and (c) show how hydraulic pressure in a first speed clutch, hydraulic pressure in a second speed clutch and hydraulic pressure in a lock-up clutch vary as time elapses, while taking account of speed changing, e.g., from the first speed to the second speed, respectively.
According to the conventional control system, if a speed changing command is issued at the time t.sub.1, the first speed clutch and the lock-up clutch are turned off at this time t.sub.1 and hydraulic oil starts flowing in the clutch for second speed. As a result, at the time t.sub.1, hydraulic pressure exerted on the clutch for first speed and hydraulic pressure exerted on the lock-up clutch are reduced from a predetermined pressure to a level of zero, as shown FIGS. 28(a) and (c). On the other hand, hydraulic pressure exerted on the clutch for second speed starts gradual increasing from the time t.sub.2, after the filling time t.sub.f elapses, as shown in FIG. 28(b).
The filling time t.sub.f represents a time for which hydraulic oil is filled in an empty clutch pack at the rear stage clutch. When the clutch pack is fully filled with hydraulic oil, the filling time is terminated and hydraulic pressure in the clutch at the rear stage (clutch for second speed) starts increasing.
As the filling time t.sub.f elapses, output torque from the speed changing unit is reduced to a level of zero, as shown in FIG. 28(d). The reason why such reduction occurs is attributable to the following two factors.
The reason why the lock-up clutch is released during speed changing consists in reducing load to be carried by the speed changing clutch or suppressing consumption of energy generated by the engine. With the prior art, however, the lock-up clutch is turned off at the same time when the clutch at the front stage (clutch for first speed) is opened (time t.sub.1). While the lock-up clutch is turned off, engine output passes through the torque converter but, at the time of this operative state, a speed ratio of turbine to pump in the torque converter, i.e. an e value (=N.sub.t /N.sub.p) assumes 1. FIG. 29 shows a plurality of characteristic curves of the torque converter. As is apparent from the drawing, when the foregoing speed ratio e is 1, the torque converter remains within the coupling range (the working range having a torque ratio of 1) but does not remain within the converter range (the working range in which torque exchange takes place). Thus, torque exchange does not take place in the torque converter at any time later than the time t.sub.1 when the lock-up clutch is turned off.
For the period of filling time t.sub.f until the clutch pack is fully filled with hydraulic oil, hydraulic pressure is not raised up to a level sufficient to bring the clutch in an operatively engaged state.
In this manner, according to the conventional control system, the period of time t.sub.1 to t.sub.2 for which output torque is reduced to zero is existent due to the above-described two factors and the foregoing period of time offers a factor of breathing at the time of speed changing or degrading a property of acceleration.
Usually, a system wherein at the time of speed changing, the lock-up clutch is completely released from the operatively engaged state (hydraulic oil in the clutch is drained, in other words, hydraulic pressure in the clutch is reduced to zero) and then pressurized hydraulic oil is supplied again (during speed changing, the vehicle runs with the use of the torque converter to reduce a load to be carried by the speed changing clutch) is employed for the conventional apparatus. This leads to such problems that time required for filling the lock-up clutch with hydraulic oil, i.e., the filling time fluctuates and a large magnitude of shock occurs when the lock-up clutch is brought in an operatively engaged state.
As described above, the conventional modulation valve 110 provides an uniformly extending firm pattern of hydraulic oil in the lock-up clutch and thereby a characteristic of gradual increasing of hydraulic pressure at the time of starting of forward movement of the vehicle or during speed changing is kept constant at all times.
When the lock-up clutch is brought in an operatively engaged state, the input shaft of the transmission is connected directly to the output shaft of the engine. This permits variation of torque of the engine to be transmitted to the output shaft of the transmission. With the conventional apparatus, however, since the lock-up clutch is supplied with hydraulic oil having a comparatively high pressure during normal running of the vehicle, variation of torque of the engine is transmitted to the output shaft of the transmission as it is left unchanged. Due to this problem, the conventional apparatus does not carry out lock-up running not only in a low speed region where the engine is rotated at a low speed but also in a high speed region where the engine is rotated at a comparatively high speed. This leads to another problem that the engine is rotated with a degraded property of fuel consumption.
The present invention has been made with the foregoing background in mind.
An object of the present invention is to provide an apparatus and a method for controlling a lock-up clutch wherein variation of the actual initial hydraulic pressure corresponding to variation of hydraulic pressure in the torque converter can be prevented reliably.
Other object of the present invention is to provide an apparatus and a method for controlling a lock-up clutch wherein a phenomenon of breathing during speed changing can be prevented by eliminating a period of time for which output torque during speed changing is reduced to a level of zero, a property of acceleration can be improved and shock caused by speed changing and locking-up can be reduced substantially.
Another object of the present invention is to provide an apparatus and a method for controlling a lock-up clutch wherein a property of fuel consumption can be improved by frequently executing lock-up running of the vehicle.